The present invention relates generally to MR imaging and, more particularly, to a method and system of imaging devices having an imageable tag that includes nuclei that precess at a Larmor frequency different than that of hydrogen when subjected to a polarizing magnetic field.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR imaging is frequently used for tracking or otherwise determining the position of an intracorporeal device, such as an endovascular catheter. Hereinafter, the term “intracorporeal device” generally refers to any type of device that is navigable, moveable, or otherwise insertable in whole or in part within a body. To properly guide the device, a number of tracking techniques have been developed. These techniques generally fall into one of two categories: passive tracking or active tracking.
Passive tracking utilizes signal voids or image artifacts for visualization of the medical device. Typically, the medical device is labeled with a paramagnetic marker. Paramagnetic markers are commonly used because the paramagnetic properties of the marker substance shorten its relaxation time. As such, with the appropriate pulse sequence parameters, a signal will not be collected from the marker resulting in a signal void in a reconstructed image.
Other passive tracking techniques include use of susceptibility artifacts on metal wires connected to the medical device. In this regard, the artifacts in a reconstructed image reflect the presence of the medical device. In a further passive tracking technique, electrical current is induced in the electrical wires during signal acquisition so as to modify the intensity of the artifacts for improved device detectability. Additional passive tracking techniques include use of intravascular contrast agents or the passing of similar suitable fluids through a lumen. Passive tracking of devices, however, does have drawbacks.
While passive tracking supports the simultaneous visualization of endovascular devices and subject physiology, such as blood vessels and surrounding tissue, the spatial and temporal resolutions are acquisition dependent and, as a result, the spatial and temporal resolution is inadequate to distinguish the endovascular device from subject anatomy. Further, since the markers used to tag the devices predominantly include hydrogen nuclei, it is difficult to distinguish between subject anatomy and the device with MR imaging of precessing hydrogen.
Active device tracking techniques involve the placement of an RF receiver coil on the endovascular device or use of a guide wire as a linear receiver coil. In this regard, MR signals are acquired at the endovascular device and may be used to reconstruct tracking images. While active tracking techniques are commonly preferred because of the high signal-to-noise ratio (SNR) as well as higher spatial and temporal resolution it provides, electrical wires connect the RF receiver coil to the data acquisition system of the MR scanner. These electrical wires add to the complexity of the endovascular device and can be cumbersome when inserting and positioning the device in the subject. Additionally, it may not be desirable to have electrically conductive leads extending from a subject undergoing an MR scan.
It would therefore be desirable to have a system and method capable of tracking a wireless intracorporeal device through a subject without sacrificing SNR as well as spatial and temporal resolution.